Field of the Invention
The present invention relates to a disk-shaped glass substrate to be made into a magnetic-disk glass substrate, a magnetic-disk glass substrate, a method for manufacturing a magnetic-disk glass substrate, and a magnetic disk.
Background Information
Nowadays, personal computers, digital versatile disc (DVD) recorders, and the like have a built-in hard disk drive (HDD) for data recording. In particular, a magnetic disk in which a magnetic layer is provided on a glass substrate is used in a hard disk drive that is used in a device premised on portability, such as a notebook-type personal computer, and magnetic recording information is recorded on or read from the magnetic layer with a magnetic head that flies slightly above the surface of the magnetic disk. A glass substrate is unlikely to be plastically deformed compared with a metal substrate (aluminum substrate) and the like, and thus is preferably used as a substrate of the magnetic disk.
Moreover, the density of magnetic recording has been increased to meet the demand for an increase in the storage capacity of hard disk drives. For example, the magnetic recording information area (recording bit) has been made smaller using a perpendicular magnetic recording system that causes the direction of magnetization in the magnetic layer to be perpendicular to the surface of the substrate. This makes it possible to increase the storage capacity per disk substrate. Also, in order to further increase the storage capacity, the distance between the recording and reproducing element portion of the magnetic head and the magnetic recording layer is made very short by causing the element portion to project farther, thus further improving the accuracy of the recording and reproducing of information (improving the S/N ratio). It should be noted that such control of the recording and reproducing element portion of the magnetic head is called a dynamic flying height (DFH) control mechanism and a magnetic head equipped with this control mechanism is called a DFH head. A magnetic-disk glass substrate that is used in an HDD in combination with such a DFH head is produced so as to have an extremely smooth main surface in order to prevent the substrate from colliding or coming into contact with the magnetic head and the recording and reproducing element portion that projects farther therefrom.
Servo information to be used to position the magnetic head on a data track is recorded on the magnetic disk. It is conventionally known that when the roundness of an edge surface of the magnetic disk on the outer circumferential side (also referred to as “outer circumferential edge surface” hereinafter) is reduced, the magnetic head flies stably, the servo information is favorably read, and thus the magnetic head stably reads/writes information. For example, JP 2009-157968A states that in the technique described therein, processing is performed under two conditions for respectively forming two undulations having different wavelengths in a forming step of forming an edge portion on the outer circumferential side of a glass substrate. Accordingly, the undulations of the outer circumferential edge surface of the glass substrate can be eliminated to improve the roundness.